Is that because you want date/times in JSON? Shouldn't it include a timezone, and maybe implement ISO8601?
1. Datetime
2. Expand the encoding to escape '<' and '>'
Being able to specify a datetime as local vs utc would also be nice... so "2018/01/31 17:35:21.2222Z" for UTC and "2018/01/31 17:35:21.2222" for local.
As to the actual format used... that it's just a bikeshed argument waiting to happen. I picked y/m/d for no particular reason (well... other than my ancient preference for a simple sort key). I also picked 24 hour time for no particular reason (well... other than my ancient preference for a simple sort key).
No one would really care too much one way or another what the format was (so long as it was easy to generate using common server side technologies... node, php, java, .net).